Geomagnetic Field, Main: Theory
The theory of geomagnetism goes back to Gilbert's book De Magnete , published in 1600, in which he proposed that the Earth itself is a magnet like a lodestone. The idea that the geomagnetic field originates from the ferromagnetism of the Earth's material prevailed until the early years of this century when it became clear that the Curie temperature of ferromagnetic materials is exceeded a few tens of kilometers inside the Earth's crust and that an unreasonably high magnetization of the upper crust would be required. Even before that time the phenomenon of secular variation was difficult to reconcile with the hypothesis of remanent magnetism.
Numerous other hypotheses about the origin of geomagnetism have been explored and disregarded (Rikitake, 1966) except for the dynamo hypothesis originally put forward in 1919 by Larmor to explain the magnetic field of sunspots. According to the dynamo hypothesis, motions within the outer liquid-iron core of the Earth